1. Field of the Invention
This invention relates to batch cleaning apparatus, and more particularly, to an apparatus for cleaning printed wiring assemblies and similar articles in a batch fashion.
2. Description of the Prior Art
Apparatus that could be used to clean articles such as printed wiring assemblies, in a batch fashion, are known in the art. "Printed Wiring Assemblies" signifies all types of assemblies whether they be referred to as printed circuit boards, surface mounted components or thick film hybrid circuit assemblies.
A device that washes baking pans and that includes a rack for supporting the articles to be washed, spray nozzles and means for moving the nozzles between the rows of articles is shown and claimed in U.S. Pat. No. 3,598,130 issued Aug. 10, 1971 to Nolte and others. However, the flow of water from the nozzles of the Nolte device is not a directed flow. Thus, the water could not be directed into the very narrow spaces, of the type existing on printed wiring assemblies, between the back side of the electronic components and the adjacent surface of the assemblies.
U.S. Pat. No. 3,868,272, issued Feb. 25, 1975 to Tardoskegy, discloses a cleaning apparatus where printed circuit boards, during the cleaning operation, are supported on an endless traveling screen or perforated metal belt so that the boards are subjected to the action of cleaning fluids directed to both sides of the boards. However, this apparatus has no reciprocating spray header with nozzles capable of cleaning boards arranged in a stationary position. Consequently, its cleaning action is very limited.
U.S. Pat. No. 3,928,064 issued Dec. 23, 1975 to Holm discloses a method of cleaning plate-shaped objects by moving the objects past a tub containing a cleaning liquid and a plurality of brushes. This method is not effective to properly clean modern printed wiring assemblies, in particular, inside the narrow space between the electronic components and the adjacent surface of an assembly. The brushes are inadequate to thoroughly clean such narrow spaces.
British Pat. No. G B 2 063 414 A by Electrovert Limited, published on June 3, 1981, discloses a device having high velocity jets of cleaning liquid discharged from nozzles disposed above and below printed circuit boards carried by means of a conveyor. The boards are subjected to an additional cleaning action of liquids directed horizontally toward the boards and positioned alongside the conveyor. Nevertheless, the nozzles remain stationary and the boards move by means of a conveyor. In this device the cleaning action is limited since the jets of cleaning fluids are unable to reach every portion of the upper and lower surfaces of a board. While advancing on a conveyor, the boards are exposed only once to the cleaning action of the nozzles. Thorough cleaning and drainage of modern printed circuit boards can not be obtained with this device.
The electronic industry trend toward surface mounting of miniaturized components is creating a demand for better cleaning methods. The latest technologies made possible the manufacturing of assemblies with extremely small space between the electronic components and the adjacent surface of an assembly. This accomplishment imposes a significant hindrance to the effective cleaning of printed wiring assemblies with conventional devices. Clearly, the art of increasing the density of electronic circuits has outdistanced the art of cleaning such densely mounted circuits. Consequently, there is a present need for an apparatus capable of cleaning even the most densely packed modern printed wiring assemblies.
There are other factors that intensify the demand for perfection in the cleaning of printed wiring assemblies. The ionic and organic residues originated in the assembly stage become very destructive when activated by moisture and/or thermal electromigration. This results in chemical and galvanic corrosion and in vesication. Vesication, also known as mealing, is the formation of blisters between the interface of the protective conformal coating and the printed wiring assembly surface. By proper assembly cleaning and elimination of organic residues the moisture penetration may be sufficiently reduced. This is necessary for long term storage and operational requirements of assemblies since conformal coating is not considered a true hermetic seal.
It is, accordingly, a general object of this invention to provide an apparatus that can thoroughly clean and dry even the most modern printed wiring assemblies.
Another object of this invention is to provide an apparatus that can clean printed wiring assemblies in a batch fashion, as distinguished from the widely used conveyor belt methods of the prior art.
Another object of this invention is to provide an apparatus that can remove the ionic and nonionic residues of printed wiring assemblies.
Still another object of this invention is to provide an apparatus that can remove organic residues from printed wiring assemblies.
A further object of this invention is to provide an apparatus with jet streams capable of reaching the very narrow spaces between the back side of the electronic components or chips and the adjacent surface of a printed wiring assembly.
Still another object of this invention is to provide an apparatus with nozzles that discharge high impact, sharply defined, flat-fan pattern jet streams.